1. Field of the Invention
This invention relates generally to an apparatus for reproducing digital signals, and more particularly an apparatus of this kind which is suitable for use in reproducing digital signals recorded on a tape by rotary heads.
2. Description of the Prior Art
There have been proposed various apparatus for recording and/or reproducing PCM coded audio and video signals in and from oblique tracks formed on a tape by rotary heads.
In general, such rotary head equipped recording and/or reproducing apparatus are provided with a tracking servo so that the rotary heads correctly scan the recorded tracks. In the above-mentioned apparatus for recording and/or reproducing PCM coded signals, the tracking servo uses a control signal recorded, for example, on an end portion of the tape in its width direction and a pilot signal recorded in tracks for recording PCM-coded signals by frequency division or area division. For example, the tracking servo using the pilot signal is used in the recording and reproducing circuit for 8 mm video tape recorders and digital tape recorders equipped with rotary heads (hereinafter simply referred to as "R-DAT").
However, in using the tracking servo it is necessary to adjust with high accuracy the position of the head in its so-called head height direction, i.e. perpendicular to the rotating direction of the rotary heads, and also an angular distance between the heads, that is, a so-called angular division ratio in the case of apparatus equipped with two heads, thereby requiring an extremely expensive drum on which the rotary heads are mounted.
Particularly in the case of the format for the R-DAT, the location of areas in which the pilot signals for the tracking servo are recorded is complicated and accordingly high accuracy is required therefor.
Further, in a conventional apparatus the drum and the capstan have to be servo-controlled independently of each other so that they cannot be driven by the same motor, resulting in a higher production cost thereof.
By way of background, the PCM signal is recorded in the following manner:
A plurality of blocks of PCM signals form one frame. A block address and an address of the frame to which the block belongs are added to the blocks. Next, each frame unit is interleaved and then an error correcting code and so on are added to the interleaved frames. Finally, each of the frames is recorded in one or two oblique tracks formed on a tape.
The PCM signal thus recorded is reproduced in a manner such that the block data having the same frame address is sequentially written into a buffer RAM for processing PCM signals for reproduction in accordance with the block addresses, subjected to processing such as deinterleaving, error correction and so on, and then reproduced.
It can be noted that if several frame portions of a PCM signal can be written into the buffer RAM, the PCM signal can be reproduced without high tracking accuracy.
Thus, there has been proposed a so-called non-tracking servo system which does not rely on a tracking servo of the type which uses a control signal and a pilot signal as mentioned above. Such a system is disclosed in, for example, copending U.S. patent application Ser. No. 087,093, filed Aug. 19, 1987.
In this case, however, the tape transporting speed upon recording may not be equal to that upon reproducing due to differences in various conditions. If no correction is made, data recorded in the same track will be reproduced twice, the track will be jumped to reproduce data in the next track, or the like. Thus, it is required to reproduce the signals at the same tape speed as upon recording.
Japanese Laid-open Patent Application No. 61-39961 discloses an apparatus for controlling the tape transporting speed in response to the difference between a write address for writing a reproduced signal from a reproduced block address into an RAM and a read address for reading a written signal from the RAM.
However, according to the above-mentioned Japanese Laid-open Patent Application, the frame address, which varies corresponding to rotation of the rotary heads, is not recorded on the tape so that the tape transporting speed can not be controlled by the use of the frame address.